I. Field of the Invention
This invention relates to apparatus for refurbishing arcuate surfaces, such as the overlay weld used in joining straight lengths of pipe to nozzles commonly found in nuclear boiling water reactor systems, and more particularly to a milling machine which can be rapidly and accurately put into position and which is capable of following the arcuate profile of the overlay weld to reduce the exposure time of workers to nuclear radiation.
II. Discussion of the Prior Art
Nuclear boiling water reactor systems incorporate stainless steel pipes having recirculating nozzle structures made from high carbon steel joined thereto where the nozzles are outwardly divergent and define an arcuate surface profile proximate their point of joinder. The nozzle is welded to the pipe at this location. Weld overlay is frequently used as a temporary fix for a condition referred to in the industry as intergranular stress corrosion cracking or IGSCC. Because of the high heat, radiation and fatigue environment found in nuclear boiling water reactors IGSCC often occurs at the weld interface. This condition must be remedied to avoid leakage of radioactive water. The eventual remedy involves removal of the cracked overlay weld material and subsequent replacement with new pipe and weld. The method of the prior art used to remove the overlay weld involved the use of abrasive grinding in a totally manual operation. Because the pipes and nozzles are within the dry well of the nuclear boiling water reactor, it is a high radiation environment and, hence, federal regulations permit only limited time exposure, necessitating a large crew. Moreover, the grinding operation generates airborne contamination in the form of radioactive metallic dust particles.
Therefore, a need exists for a machine which can be used to accurately and precisely remove an overlay weld having an arcuate profile and which minimizes the length and degree of exposure of the workmen to atomic radiation hazard.
In my earlier U.S. Pat. No. 4,739,685, I described a split frame clamshell-type portable machining lathe which is adapted to encircle a pipe and which is used to drive a tool bit in a circular orbital path about the pipe while advancing the tool bit in the radial direction. This allows a radial groove or cut to be made through the wall of the pipe. Because both the clamshell lathe and the tool advance mechanism are power driven, the only manual effort involved is the initial setup of the clamshell lathe on the pipe to be worked upon and its subsequent removal when the task has been completed. The machine of my aforereferenced patent greatly reduces the exposure time of operators to the radiation environment, especially when contrasted to earlier methods when pipes had to be cut to length using a hacksaw.
In accordance with the above-referenced patent and with my further U.S. Pat. No. 4,944,205, the tool bit is made to move only in the radial direction and, therefore, cannot effectively be used to remove an overlay weld which may extend five inches or more in the axial direction.
From the above, it can be seen that there is a need for a machine which may be rapidly mounted on a pipe and which can be used to machine off an arcuate overlay weld in a nuclear boiling water reactor with a minimum of human intervention. It is also desirable that this be accomplished without creating airborne radioactive metal particles (dust) which might later be spread by persons who are exposed to that dust and carry it on their clothing.